This invention relates to apparatus for controlling the flow ratio of two gases, each having a separate supply. It is particularly useful where a main gas is to constitute more than 95% of the flow and where the second gas is to constitute less than 5% of the flow. A specific example of where the invention has special usefulness is in controlling the carbon dioxide ratio to oxygen for a breathing gas supplied to a ventilator.
Clinical experience indicates that prolonged mechanical ventilation of patients tends to decrease arterial carbon dioxide tension below the normal levels. This condition known as "hypocapnia" may have undesirable effects on arterial oxygen tension, cardiac output, cerebral blood flow, release of oxygen from hemoglobin to the tissues, and other such parameters.
In the prior art the attempts to prevent hypocapnia have relied on increasing the capacity or mechanical dead space in the breathing circuit of the ventilator so that a portion of the carbon dioxide released by the patient during his exhalation was added to the volume of breathing gas delivered by the ventilator during the inspiratory phase. However, it has been found that this prior art system was difficult to adjust properly and did not provide good control of the carbon dioxide tension.
In the present invention a new apparatus enables much more accurate and controlled delivery of a proper amount of carbon dioxide to control the carbon dioxide tension. The apparatus senses the flow of the breathing gas to be delivered to the patient via the ventilator, and it adds to this breathing gas, on an instantaneous basis, a flow of carbon dioxide so that a predetermined percentage of carbon dioxide is automatically maintained in the breathing mixture.
Thus, one object of the invention is to provide a good control of the ratio of carbon dioxide to oxygen in a breathing gas.
Another object of the invention is to enable accurate ratio control between any two gases, especially where one gas is supplied in an amount considerably greater than the other.
Another object of the invention is to improve the control of hypocapnia.